#Et weather calculator manual#

Model E can be connected to any event data logger such as Irrometer watermark datalogger, Aquatrac etc.įor more information on ET Gages visit. The cost of Model A is $355 and Model E is $700.

#Et weather calculator manual#

In the manual model (Model A), the site tube has to be read manually once or twice a week. However, the electronic Model E automatically senses each time there is a 0.01-inch evaporation from the ET Gage. This device comes in two models, manual (Model A) and electronic (Model E). Reading a site tube is as easy as reading a rain gauge. For each inch of water used by the plant, the water level in the site tube (shown in Figure 2) drops by one inch. The reservoir at the bottom of an ET Gage is filled with distilled water. For example, #30 cover simulates grass reference evapotranspiration (ET o) and #54 cover simulates alfalfa reference ET (ET r). There are different canvas covers available and can be changed for different plant surfaces.

It has a canvas covered ceramic evaporator plate at the top that allows water to evaporate the same way as a crop does. One such tool is the atmometer or ET Gage (Figure 2).ĮT Gage is a device that simulates the crop water use from a plant canopy.

Since there are not many weather stations available in Minnesota that provide ETc information, there are alternative tools to determine ET ref. Mean crop coefficients (Kc) of commonly grown crops in Minnesota for use with alfalfa reference ET.įigure 2. Daily ET estimates (for SE Minnesota and Wisconsin).NDAWN Crop water use (NW Minnesota and North Dakota).Central Minnesota Agricultural Weather network - This website provides daily crop water use information for different cropping systems that can be used in water balance irrigation scheduling.Climatic data included are air temperatures, solar radiation, humidity and wind. Climate variables measured by a weather station are used as input data to calculate ET ref. Crop coefficient values for common crops grown in Minnesota are given in Tables 1 -3.Īlfalfa-reference and grass-reference evapotranspiration (ET ref) are calculated using the standardized Penman-Monteith equation. Where ET ref is the reference evapotranspiration - which is the evapotranspiration from a reference crop (grass or alfalfa) - and K c is the crop coefficient. Discussions about MAD, AWC and TAW are available in Basics of irrigation scheduling. More information about MAD strategies and pumping capacity can be found in Irrigation management strategies.Ĭrop Evapotranspiration (ET c) can be estimated using a two-step approach to determine the crop-specific water use (ET c) as per the following equation: However, if the irrigation system has limited capacity, then the irrigator should not wait for Dc to reach MAD and should irrigate more frequently. Generally, irrigation should be initiated when Dc approaches MAD. Plants start to experience water stress once the soil water deficit/depletion in the root zone is greater than the root zone MAD. Remember that TAW = Available water holding capacity (AWC) X rooting depth.

The estimated soil water deficit (Dc) from the water balance equation is then compared with maximum allowable depletion (MAD) - which is usually 50% of total available water (TAW) in the root zone - to make irrigation decisions. From the initial soil moisture content, soil water depletion/deficit (Dc) for the successive days can be estimated using equation 2. You can estimate initial soil moisture using gravimetric soil water sampling, the hand-feel method or soil moisture sensors. Using these assumptions, equation 1 can be simplified to:ĭ c = D p + ET c - P - Irr (Equation 2) Estimating initial soil moisture and soil water deficitīefore beginning the water balance calculations, you should know the initial soil moisture. Since it is very difficult to estimate R and DP in the field, these variables can be accounted for by setting Dc to zero whenever water additions (P and Irr) to the root zone are greater than water subtractions (Dp + ET c). Where Dc stands for soil water deficit (net irrigation requirement) in the rooting zone on current day, Dp is the previous day soil moisture deficit, ET c is crop evapotranspiration on the current day, P is precipitation for the current day, Irr is the irrigation amount for the current day, R is the surface runoff and DP is the deep percolation. Daily soil water depletion in the rooting zone is calculated using the equation below:ĭ c - D p = ET c - P - Irr + R + DP (Equation 1) Outputs include ET c, runoff (R) and deep percolation (DP). Major inputs include precipitation (P) or rainfall and irrigation (Irr). Estimating soil water using the water-balance approach is done by accounting for all the incoming and outgoing water from the soil root zone (Figure 1).